We will study the process of neurosecretion in the caudal neurosecretory complex (CNC) of the teleost fish, Gillichthys mirabilis. This system, found in the posterior portion of the spinal cord, produces at least two peptide hormones, urotensins, which have not been described in other tissues. The function of urotensin I (UI) in fish is uncertain; however, in birds and mammals it has a potent vasodepressor effect. The structure of this hormone has not been fully determined. We propose to complete the chemical characterization of this hormone. Urotensin II (UII), which we have recently shown to be structurally similar to somatostatin, is involved in osmotic balance in the fish and has been implicated in several other physiological roles. We have used high performance liquid chromatography to develop a rapid isolation procedure for this hormone. The unique morphology of this system and our ability to maintain the intact CNC as an organ culture for periods of several weeks will allow us to use this system to study the various stages of the neurosecretory process. We will examine the stimulus-specific release of UTI by incubating the CNC with suspected effector compounds (acetylcholine, norepinephrine) or by altering its ionic environment while monitoring hormone release with a radioimmunoassay. We will study biosynthesis of UII by incubating the CNC in the presence of radioactive amino acids and subsequently determining the specific radioactivity of the isolated hormone. By isolating hormone labelled in vitro separately from the spinal cord (the presumed site of synthesis) and the urophysis (the storage and release organ) we will obtain information about axonal transport of the hormone.